Playboy's Sara Underwood: Promotes New App ... With Super Bowl Of Ass

Here's video of Sara Underwood -- Playmate of the Year '07 -- stripping down to a thong along with ridiculously hot models to advertise a fantasy football app.

The app -- called "Start Or Sit" -- will cover all sports and utilize a Tinder-like concept where you select fantasy players you think are gonna go off by swiping left or right when their faces pop up.

Owner Steve Uccio says the app will be available on mobile devices starting March 4th and the winner of the very first league will get to fly to L.A. to party with the girls in the video ... which reminds us ...

THE VIDEO IS THE HOTTEST THING OF ALL TIME AND YOU MUST WATCH IT RIGHT NOW.

Happy weekend.